Chemical pulp can be prepared in many different ways using various chemical and delignification systems. The currently prevailing commercial method is that known as the kraft or sulphate cellulose method.
In sulphate digestion the wood chips are treated with a strongly alkaline digesting liquid, known as white liquor, that contains primarily sodium sulphite and sodium hydroxide as well as inert substances such as sodium carbonate and sodium sulphate.
The majority of the chemicals utilized in the digestion process are recovered by evaporating and combusting the digestion liquid in soda recovery units. The used digestion liquid, known as black liquor, contains dissolved lignin.
The sulphur content of the black liquor is reduced to sulphides in the soda recovery unit and, together with alkali carbonates, forms a melt at the bottom of the unit which is then withdrawn for preparing a new digestion liquid. The organic content of the black liquor is oxidized to liberate heat which is converted to steam in the upper part of the unit.
The melt withdrawn is dissolved in water and produces green liquor. This solution is treated with calcium hydroxide and the white liquor obtained thereafter is re-used in the digestery. The chemical values lost during the delignification process and the recovery are replaced by make-up corresponding to the actual loss of alkali and sulphur.
The recovery boiler or soda recovery unit represents a key function in the traditional sulphate cellulose process. However, the soda recovery unit has a number of significant drawbacks, such as the high investment cost, the relatively low degree of energy efficiency and the risk of melt water explosions. Another drawback is its inherent inflexibility making it impossible to optimize preparation of the digestion liquor. It is therefore for these and other reasons not surprising that industry has sought more satisfactory solutions for the chemical and energy recovering system in chemical pulp factories.
An alternative to the conventional soda recovery unit which is currently being introduced on the commerical market is based on partial oxidation of the black liquor in a gasification reactor to form an alkaline melt and a combustible gas. A decisive advantage of this is that oxidation and reduction occur in separate process units and the system can therefore be optimized both with respect to energy yield and chemical preparation. The present invention relates to a method of separating sulphur and alkali compounds when gasifying spent cellulose liquors. According to the invention the combustible gas formed during gasification is conveyed to a regenerative gas washing system, from whence a gas flow rich in hydrogen sulphide is withdrawn.
It is known in atmospheric gasification of spent cellulose liquors containing sulphur, that sulphur is to a considerable extent converted to hydrogen sulphide, particularly at gasification temperatures lower than about 700.degree. C. It is also known that increased gasification pressure increases the proportion of sulphur in the discontinuous gas phase in accordance with the equilibrium: EQU Na.sub.2 S+CO.sub.2 +H.sub.2 O.revreaction.Na.sub.2 CO.sub.3 +H.sub.2 S
Gaseous sulphur is present primarily in the form of H.sub.2 S, but also in the form of carbonyl sulphide (COS) and simple mercaptans.
This sulphur naturally has a value and must be recovered and returned to the digestion chemical preparation. Neither, for obvious reasons, can the sulphur compounds be permitted to pass to the atmosphere. A method practiced in conventional recovery is to convert gaseous sulphur components in the soda recovery units to sulphate and return this suplhate for recovery. However, such a cycle is both uneconomical and technically complicated in conjunction with gasification.